The present invention pertains generally to electronically scanned antennas and more particularly to scanned directive antenna arrays. An outstanding problem in naval fire control is the simultaneous tracking of multiple targets. This problem is partially solved by the use of electronically scanned antenna arrays. However, due to the complexity (mutual impedance and complex computerized steering commands) and resultant cost of these systems, their use has been fairly limited in the Naval Fleet.
Another configuration which has been considered is a series of single radar beams produced by directive antenna elements which are selectively addressed in accordance with their placement in space to effectuate a steered output beam. End fire antenna elements, e.g., dielectric rods, have advantages over alternative beam directors such as parabolic reflectors, lenses, and antenna subarrays since they occupy considerably less cross-sectional surface area than the others. The physical length of the end fire antenna elements however have virtually eliminated their use.
In studying the physical characteristics of end fire rod antennas, diffraction theory indicates that if D represents the maximum antenna diameter and .lambda. the responsive antenna wavelength, then the minimum angle .theta. of the antenna beam within which radiation can be concentrated is proportional to .lambda./D. Therefore, to achieve small angles, .lambda. must be small and D large. However, both .lambda. and D are constrained by other system characteristics.
The wavelength .lambda. is basically restricted in radar to a limited range of wavelengths. Therefore, the only method of restricting the angle .theta. is to increase D. By making D large in a discrete elemental linear array (D is now the length of the linear array) and phasing the array for end fire (i.e., lining up a series of dipole elements and phasing each successive dipole by 90.degree. so that the beam is emitted along the line of the array), the cross-sectional dimension of the array is made independent of D (length of the array) and is only restricted by the size of a single antenna element. This is usually on the order of a wavelength or less which, for I band, is about 3 cm.
Dielectric rods are ideal substitutes for the discrete elemental linear array phased for end fire since they can be easily phased for end fire due to their inherent physical characteristics, they can be constructed of any one of a number of low loss materials available, and they are easily matched for impedance over a wide range of frequencies.
However, the half power beam width (HPBW) of a dielectric rod antenna indicates that: ##EQU1## Using I band (.lambda. = 3 cm), a 6.degree. HPBW requires a rod of approximately 3 meters (.perspectiveto. 10 ft.). Evan if the Hansen-Woodyard supergain relation is applied, a "10 foot pole" would only produce a 4.degree. beam or could be reduced to a "7 foot pole" to retain a 6.degree. HPBW beam which is still clearly unsuitable for use in an array.
Even if a smaller directive antenna were available, a conformal scanning system would be needed to address the directive elements. Such a system would be required to provide a simple and inexpensive method for selectively addressing directive antenna elements of the array to provide an orderly manner of scanning in three dimensions.